1. Field of the Invention
This invention relates to the testing of metal components for cold compression of the metal and finds particular use in the testing of metal plate or panels having apertures formed through them treatable for the prevention of fatigue crack propagation by cold expansion of the metal surrounding the apertures.
2. Description of the Prior Art
It is well known to provide fatigue enhancement of metal aircraft components having apertures in them. Such fatigue enhancement is obtained by "cold expansion" of the aperture by drawing an oversized mandrel through the aperture, usually with a split sleeve positioned between the mandrel and the wall of the aperture to avoid frictional interference between them.
The split sleeve method of cold expansion involves positioning a hardened sleeve with a split along its length over a tapered mandrel and positioning both inside the aperture. The tapered mandrel is then drawn through the split sleeve which is thereby expanded to provide a compressive stress in the metal surrounding the aperture. The aperture becomes permanently slightly enlarged. A permanent compressive stress zone is thereby created around the aperture made up of both plastic and elastic strains. The elastic component is what gives the fatigue protection and the plastic component causes the elastic component to be permanent.
Aperture expansion by the split sleeve method leaves a witness mark at the position of the sleeve split and a small volcano ridge around the exit side of the aperture on a surface of the component which the aperture breaks. These are clear indications that the aperture has been expanded. However, these marks, and ridges subsequently have to be removed by reaming and de-burring to enable fitting of a fastener into the aperture. Detection of the cold expansion process thereafter is thus rendered extremely difficult. It will be appreciated by those skilled in the art that in components where structural strength per unit weight is paramount, for example aircraft, and where the metal of the component has been worked to provide maximum strength, if for some reason this calculated strength is diminished then the component is liable to suffer early failure in service.
For some time a method of detecting whether such metal components have been cold worked after operations such as reaming and de-burring has been sought without success with the risk that unwanted crack propagation from fastening apertures in metal components such as aircraft wing structures has been known to occur due to fatigue loading in service.